Well known in the art are formulations of volatile phosphate esters which release vapors of the ester slowly at controlled rates to provide sufficient ester in the environment surrounding the formulation to control insects therein over an extended period of time. Examples of such formulations, commonly known as controlled-release formulations, are described in U.S. Pat. Nos. 3,076,744, 3,318,769, 3,169,416 and 3,223,513 and Canadian Pat. Nos. 649,759, 701,470 and 755,683, in which volatile phosphate ester pesticides are formulated in a thermoplastic resin or wax substrate. Other materials, such as thermosetting resins, absorbent papers, clays and other materials have been proposed as the substrate.
In using such formulations to control insects, the formulation is placed in the environment -- such as a room or other enclosure -- in which the insects are to be controlled. It emits pesticide vapors. A part of those vapors are lost -- some being dissipated physically as by loss through open doors or windows or by drafts -- and some being lost chemically -- the phosphate esters react with moisture in the environment forming products which have little or no pesticidal effect. The formulations are designed to provide more vapors than the amounts that are lost so that in time the formulations provide, and for an extended period maintain in the environment, a pesticide level that effectively kills insects therein. However, typical controlled-release formulations emit the pesticide vapors at a steadily decreasing rate so that eventually the pesticide loss rate exceeds the rate at which the pesticide is emitted form the formulation and the level of pesticide vapor in the environment falls below the concentration needed to control insects therein. This point marks the end of the effective life of the formulation and it is common practice to consider that the pesticide has been depleted from the formulation when this point is reached.
It has been difficult for a householder or other user of such a formulation to know precisely when the useful life of such a formulation has come to an end. First, the "end" comes gradually, and without any apparent change in the appearance of the formulation, or any abrupt change in its properties. Second, since such formulations ordinarily are used under a variety of conditions such as temperature and extent and pattern of air flow about them, etc., and these conditions can change considerably during use, it is difficult to generalize and predict or estimate in any particular case precisely when the "end" will come.
Determination of the "end" is further complicated in the case of such formulations which are associated with means (such as containers) which permit the flow of the pesticide vapors from the formulation to be halted at will -- i.e., "on-off" formulations. To enable the user to have some estimate of when the insecticide has been depleted, it is necessary for him to keep an account of the time the formulatiom is "on."